Finally had an unused Raspberry Pi to try something out that has been on the list of "projects" for some time -transmitting with a Raspberry Pi.

After loading up a fresh image of a stripped down version of the Raspbian operating system, I installed the necessary requirements to get rPiTX to work. For more background at a high level as well as more advanced from the code side are two helpful links:

This iteration of the application is more advanced work done under the project called RPIFM. Evariste's application is more advanced as it allows the user to modify values of I and Q which create all possible modulation schemes, such as AM, FM, SSB and all the various data modes.

If you already have some basic understanding of the Raspberry Pi and associated terminology navigating via a command line interface, here is a short set of instructions I compiled to get you started. The only thing not covered is how to get an operating system on to your Pi as well as get it connected to the internet.

In my case , the raspberry pi used for this was set up for remote access over Wi-Fi using SSH.

Here is an example to get the RPITX to do something. In this case, transmit a carrier on a frequency of 146.520MHz.

Once you run the usage command and hit enter, you will get an output like this:

If you have a receiver set up on the frequency, mode and any audio or data input specified, you should hear it right away.

It is advisable to connect some form of filtering and antenna to the GPIO Pin #12 on the P1 header to minimize any interference. Before attempting to transmit, be sure you have the correct license to do so on frequency selected.

Without using any form of antenna connected to the Raspberry Pi, It seems like a range of about 50-80 feet away, depending on frequency is possible and probably further than that.

With even a simple wire for an antenna, great range can be realized. Output is not uniform across the potential range for the Raspberry Pi. Maximum output seems to be in the low to mid VHF range and not much above 700MHz or below 80MHz. Here are some of the tests I ran with no antenna on select amateur radio bands.

The lower negative number in dBm is stronger output. Tests were performed using a telescoping ¼ wave antenna on the 2M, 1.25M, 70CM and 33CM bands feeding a calibrated wide band all-mode receiver. Range from transmitting Pi to antenna was about 50 feet and through a few walls. The 2M set up was used for 6M and 10M due to the size of a 1/4w antenna and for 10M especially not having high ceilings and it being too cold to use the outside testing antenna range.

Transmitting a carrier as well as audio on AM and FM was tested. For AM and FM, sending a recorded audio file, a solid tone as well as 1200baud were tried and worked fine. On SSB, it seemed that the carrier insertion was slightly different on LSB compared to USB. This may or may not be due to the instability of the Raspberry Pi. A tone was used to get continuous modulation on SSB.

While not yet tested, using a wide band amplifier as found on E-Bay would extend transmit range with it being extremely advisable to use band pass, low pass and/or high pass filtering to maintain spectrum purity and cut down on off frequency interference at harmonics of transmitting frequency.

Possible applications for further exploration as time permits using the Raspberry Pi as an exciter or low power transmitter include:

The Raspberry Pi used for this was the Model B+. I would like to see what can be accomplished with the Raspberry Pi Zero, which has been sold out though. This is the $5 Pi, whereas the B+ can be had for as low as $20 if you look around carefully.

Its great to use a HackRF, HackRF Blue or higher end SDRs, but this could be the cheaper way to roll up your own transmitter for a fraction of the cost. Just need to apply a little knowledge.

For exciter filtering, looks like Mini Circuits has a variety of SMD band pass filters that might be good for the Pi or to use with the HackRF type of devices. Very easy drop in type of device with no discrete components needed for this application perhaps.

Update on this project, got everything working - sort of. Can access repeaters that do not have a PL tone, which are not many. Right now a bug exists on the KC2DAA repeater on Mt. Beacon I NY that kills off the need for a tone after a power outage.

Not sure how to continuously inject a PL tone simultaneously along side audio to keep a repeater open, so have been using a computer based tone generator mixed in through the computers sound card before getting it into the USB soundcard am using with the Pi.

Audio is limited and modifying the AlsaMixer on the Pi does not have much of an improvement.

The USB sound cards ordered were of poor quality.

Will try a better USB sound card and reevaluate the PL tone situation as time permits.